The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for performing fat-water separation with the ability to measure a full dynamic range of fat fraction values.
MRI uses the nuclear magnetic resonance (“NMR”) phenomenon to produce images. When a substance such as human tissue is subjected to a uniform magnetic field, such as the so-called main magnetic field, B0, of an MRI system, the individual magnetic moments of the nuclei in the tissue attempt to align with this B0 field, but precess about it in random order at their characteristic Larmor frequency, ω. If the substance, or tissue, is subjected to a so-called excitation electromagnetic field, B1, that is in the plane transverse to the B0 field and that has a frequency near the Larmor frequency, the net aligned magnetic moment, referred to as longitudinal magnetization, may be rotated, or “tipped,” into the transverse plane to produce a net transverse magnetic moment, referred to as transverse magnetization. A signal is emitted by the excited nuclei or “spins,” after the excitation field, B1, is terminated, and this signal may be received and processed to form an image.
A method for water-fat separation known as IDEAL was developed for imaging spin species such as fat and water. As described in U.S. Pat. No. 6,856,134 issued on Feb. 15, 2005 and entitled “Magnetic Resonance Imaging With Fat-Water Signal Separation,” the IDEAL method employs pulse sequences to acquire multiple images at different echo times (“TE”) and an iterativeleast squares approach to estimate the separate water and fat signal components. In the original description of the IDEAL method, the fat signal was modeled as having one resonant frequency, as did all other Dixon methods. However, recently, a multi-peak IDEAL method that models the fat spectrum as having multiple resonance frequencies was developed, as described in U.S. Pat. No. 7,924,003, which is herein incorporated by reference in its entirety.
Multi-point water-fat separation methods, from the early Dixon methods to more recently IDEAL algorithms, all must address the intrinsic challenge of water-fat ambiguity. This ambiguity problem arises due to the fact that the signal behavior of two chemical species with a single NMR spectrum, but at different chemical shifts, may appear identical in the presence of B0 inhomogeneities. In this situation, the signal from a voxel containing substantially only one species can be identical for two possible scenarios. First, where the voxel contains a first species, such as water, with a given B0 off-resonance value, and second, where the voxel contains a second species, such as fat, with a different B0 off-resonance value. For example, with water and fat, a voxel containing only fat is very similar to a voxel containing only water that is off-resonance by approximately 217 Hz at a B0 field strength of 1.5 T. This ambiguity is the fundamental challenge of chemical shift based chemical species separation, and for water and fat is, therefore, commonly referred to as the “water-fat ambiguity.” Such ambiguities often result in water-fat swaps, in which a voxel containing, for example, water is mischaracterized as containing fat.
The challenge of water-fat ambiguity is commonly addressed by assuming a slowly and smoothly varying B0 field inhomogeneity map, or “field map,” without abrupt discontinuities, or “jumps,” in the field map that would occur when there is a water-fat swap. Previous multi-echo water-fat separation methods attempted to resolve the water-fat ambiguity by enforcing field map smoothness. However, these algorithms are typically based on variations of region growing algorithms, and are similar in principle to two-dimensional phase unwrapping methods, which are well known to be error prone and sensitive to noise and the physical characteristics of the object, such as spatially discontinuous regions.
It would therefore be desirable to provide a system and method for performing water-fat separation and fat quantification that is free from ambiguities and that is insensitive to magnetic field inhomogeneities.